A cell membrane presents a formidable barrier between a cell cytoplasm and its external environment. Cells are generally impermeable to small molecules, proteins, and nucleic acids. Some small molecules can diffuse across the cell membrane, but the rate of diffusion often is too slow to be useful.
Reagents and methods exist to deliver small molecules, proteins, and nucleic acids to an intracellular compartment of a cell. Examples of such reagents and methods include lipids, calcium phosphate, DEAE dextran, electroporation, gene gun particle bombardment, recombinant viral infection, and direct microinjection. Most current reagents and methods are either toxic to cells or result in only a few cells receiving the small molecule, protein, or nucleic acid. In addition, current reagents and methods are not practical for in vivo delivery of small molecules, proteins, or nucleic acids to cells.
Some peptides have an ability to cross the cell membrane and enter a cell. These peptides, termed “protein transduction domains” (PTDs), can be linked to a cargo moiety and can transport the cargo moiety across the cell membrane and into the cell. Such transport is termed “peptide transport” because the peptides transport cargo moieties across the cell membrane and into the cell. Cargo moieties can be small molecules, proteins, or nucleic acids.
Peptide transport provides an alternative for delivery of small molecules, proteins, or nucleic acids across the cell membrane to an intracellular compartment of a cell. One well characterized protein transduction domain (PTD) is a tat-derived peptide. Frankel et al. (U.S. Pat. No. 5,804,604, U.S. Pat. No. 5,747,641, U.S. Pat. No. 5,674,980, U.S. Pat. No. 5,670,617, and U.S. Pat. No. 5,652,122) demonstrated transport of a cargo protein (β-galactosidase or horseradish peroxidase) into a cell by conjugating a peptide containing amino acids 49-57 of tat to the cargo protein.
Penetratin can transport hydrophilic macromolecules across the cell membrane (Derossi et al., Trends Cell Biol., 8:84-87 (1998)). Penetratin is a 16 amino acid peptide which corresponds to amino acids 43-58 of the homeodomain of Antennapedia, a Drosophila transcription factor which is internalized by cells in culture. However, penetratin-mediated peptide transport of nucleic acids longer than 55 bases and proteins longer than 100 amino acids is inefficient.
VP22, a tegument protein from Herpes simplex virus type 1 (HSV-1), has the ability to transport proteins and nucleic acids across a cell membrane (Elliot et al., Cell 88:223-233, 1997). Residues 267-300 of VP22 are necessary but may not be sufficient for transport. Because the region responsible for transport function has not been identified, the entire VP22 protein is commonly used to transport cargo proteins and nucleic acids across the cell membrane (Schwarze et al., Trends Pharmacol Sci, 21:4548, 2000).
There is a continuing need in the art for peptides that can efficiently transport cargo moieties across a cell membrane and into an intracellular compartment of a cell.